The present invention is concerned with an alumino-silicate type catalyst comprising an MFI zeolite, the selectivity and/or catalytic properties of which has/have been modified by desalumination of the outer surface of its crystals, and comprising at least one group VIII metal from the periodic classification of elements (Handbook of Chemistry and Physics, 65th edition, 1984-85), and a matrix. Said catalyst is used in isomerization reactions of C.sub.8 aromatic hydrocarbons. The invention is also concerned with the process for the preparation of said catalyst. Usually, the outer surface of the zeolite crystals is modified by at least one treatment of the MFI by at least one solution of a fluorosilicic acid salt. Preferably, the salt selected does not result in the formation of aluminium salts which are not soluble in water.
At the present time, the catalysts used industrially in isomerisation reactions of C.sub.8 aromatic cuts mainly have a base of ZSM-5 zeolite of MFI structural type, alone or mixed with other zeolites. These catalysts are described, in particular, in the patents U.S. Pat. No. 3,856,872, U.S. Pat. No. 4,098,836, U.S. Pat. No. 4,159,282, U.S. Pat. No. 4,467,129, U.S. Pat. No. 4,428,773, European Patent 138,617 and U.S. Pat. No. 5,028,573.
The ZSM-5 zeolite is interesting because of its selectivity of shape which gives an advantageous selectivity with respect to para-xylene and also in its selectivity with respect to undesirable secondary dismutation reactions which remain at a lower level than those recorded on other zeolites with larger pore openings. In fact, 12MR zeolites with larger pore openings (opening with 12 oxygen atoms) have also been used, such as mordenite. The catalysts with a base of mordenite are described, in particular, in the patents U.S. Pat. No. 4,723,051, U.S. Pat. No. 4,665,258, and French Patent 2,477,903. However, these zeolites do not have special properties of geometric selectivity. This means that, irrespective of their Si/Al ratio, their selectivities with respect to para-xylene are lower than those obtained for ZSM-5 zeolite, and, in particular, there is a very great amount of trimethylbenzenes produced. The production of trimethylbenzenes by dismutation is actually promoted in mordenite whose microporous system is more open than that of ZSM-5: the openings are with 12 oxygens instead of 10 for ZSM-5.